In such thread brakes which are widely used in practice, for example, in the form of so-called disc or plate brakes, the brake discs or plates forming the brake elements are usually rotatably mounted on a guide bolt having at one end a thread on which there is screwed an adjustment nut which forms the abutment of a compression spring which presses the two brake discs or plates elastically against each other. These have the inherent, fundamental disadvantage that lubricants (paraffins, bobbin oil etc.) adhering to the surface of the thread running off form deposits on the brake discs or plates and dirt particles and fluff settling in these produce a sticky, pasty mass which penetrates progressively between the brake discs or plates. In the course of time, these deposits which build up further and further during operation cause the brake discs or plates to be held apart, which makes them less and less able to exert their braking action on the thread passing through. An irregular braking effect also occurs and results in undesired fluctuations in the thread tension. In addition, the brake plates or discs are impeded in their moveability by this sticky mass, which causes the passing thread to start cutting into the brake surfaces of the brake plates or discs, a danger which is very pronounced particularly with synthetic threads. Once the braking surfaces are damaged to the extent that quite deep grooves or flutes are cut in them, the thread passing through also suffers damage.
These difficulties make it necessary for the thread brake to be cleaned and freed from undesired deposits or even exchanged altogether at certain time intervals.
To remedy this, it is known to make the brake discs or plates be driven via a gearing (German patent 27 58 334), but this involves relatively high expenditure and is only suitable in certain cases of use. Another known measure (German published patent application 30 29 509, German patent 29 30 641, to which U.S. Pat. No. 4,313,578, Van Wilson et al, corresponds) consists in using an ac-excited electromagnet instead of the conventional compression spring to press the two brake discs or plates against each other in the axial direction and simultaneously cause vibrations or oscillatory motions with twice the excitation frequency of the electromagnet to be imparted to the brake discs or plates consisting of magnetic material by the magnetic ac field. These oscillatory motions occur in the direction of the bearing axis and, independently of the oscillatory behaviour of the brake discs or plates, can result in non-uniform braking action on the thread passing through, which causes corresponding fluctuations in the thread tension. Also, in principle, such a thread brake is dependent upon an electric ac supply which, however, in many cases is not available.